<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<style>
    .box{
        width: 200px;
        height: 100px;
        padding: 20px;
        border: 5px;
        margin: 20px;
    }
    .border-box{
        /* border padding 先分配,剩余的是内容大小
           多列布局的时候有优势
        */
        background-color: lightblue;
        box-sizing: border-box;
    }
    .content-box{
        background-color: lightgreen;
        box-sizing: content-box;
    }
</style>
<body>
<div class="box border-box">Border with border-box</div>
<div class="box content-box">Box with content-box</div> 
</body>
</html>